Spoiler based on kamm-back dimensions?

[aerohead]

Originally Posted by Frank Lee

I think you'd see some gain- say, 10% vs the- say, 50% on the Pinto (turbulent wake reduction, not necessarily Cd reduction or fe increase). My understanding of it is, you can boattail quite nicely, and do things with spoilers re: that airfoil template, but wherever that bodywork ends, you still end up with a turbulent wake. Aerohead has a description of that phenomenon somewhere here.

Yours is kinda the difference between a regular minivan and an Aztek.

Frank,the best I can figure it,is that if you can reduce the wake by some percentage,then you've reduced your drag by the same percentage.It's actually probable to scale the area of the wake from the template if you measure top and sides.Wherever one chooses to cut the tail,if they figure the size of the wake at that point,compared to the original,that percentage would be the drag reduction.At 55-mph,mpg increases at 50% of drag reduction,so a 10% wake reduction should yield 5% better mpg.--------------------- The premise of my boattail trailer is to follow the template all the way out to a point,effectively reducing Profile drag to zero.I'm left with skin friction,internal drag (already cut with grille block),interference drag from side mirrors,and some lift which I'm stuck with."Proof's in the pudding" as they say,so I won't know anything 'til late September,but I really do feel as though anyone using the template can expect savings based on the wake reduction their mods provide.

[Frank Lee]

That makes sense. I figure what cf is proposing is similar in effectiveness to the kamm extension metro put on his, and what did he get- about 5% fe increase, or 10% drag decrease, which intuitively makes sense just by looking at the "kamm" rear end reduction on the template.

[aerohead]

Originally Posted by Frank Lee

That makes sense. I figure what cf is proposing is similar in effectiveness to the kamm extension metro put on his, and what did he get- about 5% fe increase, or 10% drag decrease, which intuitively makes sense just by looking at the "kamm" rear end reduction on the template.

It's funny.People seem to want to project the notion that aero is some great mystery,some "black art" which mystifies even those with stratospheric college degrees.Nature just seems to throw it out there nice and easy.That template closely resembles half of a Perrigrine Falcon,closely studied by NASA and MIT.While I will spend the rest of my life tinkering with aero,I'd sure like to see the big guns dispel all the myths and fallacies and let us move on to new business.Grade school math seems to be adequate to understand all the fine points.Weird!

[Hugh Jim Bissel]

Originally Posted by aerohead

I think that as long as the rear edge falls along the template line,that the air will re-attach,forming a locked-vortex above,and the outer flow will skip over it.It won't be as effective as the ideal Kamm roofline but you'll have good rear vision and it will be relatively easy to fab.All the air below it will be in the turbulent wake.A second wing below,and also extending to the template line would create a second locked vortex and even lower drag.Ford proved this with the 1980s Merkur XR4T1 which sports their patented bi-wing spoiler.

What have you seen, or what are your thoughts about using multiple horizontal slats for some or all of a boattail, and the spacing between the slats? (the trailing end of each slat ending at the point where the surface of a boattail would be made)

One extreme of this would be basically a sideways "T" with the crossbar at the rear of the vehicle and the bottom of the "T" where a boattail would come to a point. This would basically be the 3 foot long spoiler Frank Lee mentioned. This might help, but doesn't seem like it would be a big improvement over the stock vehicle. The next step would be something along the lines of the split spoiler on the Merkur.

The opposite extreme is paper thin slats so close together they are touching: their ends would make the solid surface of a boattail.

Since there is a boundary layer it seems reasonable that you can have the slats separated slightly with no difference between it and a full boattail; then as the slats are separated farther (fewer slats used on the vehicle) the cd will start to increase; until there is just the one "slat" (the sideways "T").

This usage of slats would allow visibility through rear window, and allow taillights to be within the boattail instead of having to Kamm the last couple feet (to mount taillights and license plate).

What I'm wondering is what thoughts/experience do you have, or what have you come across in your readings and studies as to when increasing of slat spacing distance begins to have a detrimental effect?

That is to say, slats an inch or less apart (vertically) seem like they would be close enough not to give a penalty over a boattail. A foot or more apart, however, seems like it would have a substantial cd penalty over a solid bottail. So what would be the greatest distance that causes no effect, or more likely, if spacing distance vs drag is some sort of asymptotic graph (like an inverse exponential curve), at what range does the spacing begin to have a noticeable effect?

Another way to think about it instead of slats, is pixilation: if you're building a boattail with identical small cubes, how small do the cubes need to be to make a boattail with approximately the same effect as a smooth tail?

My guess would be around 2" or so, but that's not even an educated guess (an uneducated guess? ). I'm curious to hear others thoughts and if anyone has seen any research on this.

It seems like this could potentially be a good way to have a boattail / kammback without hindering rear vision or blocking taillights (and/or giving vertical surfaces where one could mount small taillights on the boattail without adding drag)

Looking forward to hearing y'alls thoughts.

[aerohead]

Originally Posted by Hugh Jim Bissel

What have you seen, or what are your thoughts about using multiple horizontal slats for some or all of a boattail, and the spacing between the slats? (the trailing end of each slat ending at the point where the surface of a boattail would be made)

One extreme of this would be basically a sideways "T" with the crossbar at the rear of the vehicle and the bottom of the "T" where a boattail would come to a point. This would basically be the 3 foot long spoiler Frank Lee mentioned. This might help, but doesn't seem like it would be a big improvement over the stock vehicle. The next step would be something along the lines of the split spoiler on the Merkur.

The opposite extreme is paper thin slats so close together they are touching: their ends would make the solid surface of a boattail.

Since there is a boundary layer it seems reasonable that you can have the slats separated slightly with no difference between it and a full boattail; then as the slats are separated farther (fewer slats used on the vehicle) the cd will start to increase; until there is just the one "slat" (the sideways "T").

This usage of slats would allow visibility through rear window, and allow taillights to be within the boattail instead of having to Kamm the last couple feet (to mount taillights and license plate).

What I'm wondering is what thoughts/experience do you have, or what have you come across in your readings and studies as to when increasing of slat spacing distance begins to have a detrimental effect?

That is to say, slats an inch or less apart (vertically) seem like they would be close enough not to give a penalty over a boattail. A foot or more apart, however, seems like it would have a substantial cd penalty over a solid bottail. So what would be the greatest distance that causes no effect, or more likely, if spacing distance vs drag is some sort of asymptotic graph (like an inverse exponential curve), at what range does the spacing begin to have a noticeable effect?

Another way to think about it instead of slats, is pixilation: if you're building a boattail with identical small cubes, how small do the cubes need to be to make a boattail with approximately the same effect as a smooth tail?

My guess would be around 2" or so, but that's not even an educated guess (an uneducated guess? ). I'm curious to hear others thoughts and if anyone has seen any research on this.

It seems like this could potentially be a good way to have a boattail / kammback without hindering rear vision or blocking taillights (and/or giving vertical surfaces where one could mount small taillights on the boattail without adding drag)

Looking forward to hearing y'alls thoughts.

Jim,just now catching your post.Hucho's book addresses some of the points you raise.The "T" has been done( rather a "disk" behind a body of circular section).At a specific diameter and distance,it did show a drag reduction.The tough part comes when it is yawed,as we'd experience in any crosswind situation.When caught from the side,the air would trip,as with a flat plate (Cd-1.11 ) and the resulting wake would increase the original wake.----------------- The horizontal ( or longitudinal ) slats,when arrayed,stair-stepping their way such that their trailing edges always respected streamline flow,would form a series of locked- vortices of which the outer flow would skip over.---------- As to an optimized separation distance,I would have no clue.---------- One thing to bear in mind would be the strength and mass of any structure which can span the full height or width of a vehicle and not deflect.Material for material,there would be a point,where the ideal boattail would have no more mass than a series of foils attempting to outdo it.And with rear-vision cameras now,safe vision to the rear is better than ever.----------- A second point to mention would be safety to anyone accidently colliding with the slats from the rear.They would form a rather rigid structure and by default,pose a formidable face.